Typically, deep-drawn formed plastic pieces are obtained by vacuum thermoforming or pressure thermoforming by forming sheets or panels from termoplastic materials which, prior to forming, have been heated to a temperature above the softening temperature of the material. For the pressure thermoforming method, a working medium pressure of less than 6 bars is usual and quite sufficient. Heating to at least the softening temperature requires energy and time. The formed product can be removed from the mold only in a dimensionally stable state, which necessitates cooling of product and mold. Heating prior to, and cooling after each forming step requires comparatively long cycle times.
DE-A1-37 33 759 discloses a method of producing a container from a deep-drawable thermoplastic laminated sheet of plastic material, preferably polypropylene, with a polyvinylidene chloride layer. The laminated sheet is initially heated to a temperature in excess of 130.degree. C. in order to reduce the degree of crystallization of the polyvinylidene chloride, and subsequently it is quenched to a temperature of less than 75.degree. C., and directly after quenching it is vacuum thermoformed by deep drawing. Although the cooling rate should be as high as possible, a period of between 15 and 300 seconds is recommended for quenching and deep drawing.
In the method according to DE-A1-37 27 926, flat and/or preformed panels of thermoplastic materials which are unidirectionally or multidirectionally reinforced with short or long fibers are formed as intermediates by means of compression molding in a heatable mold comprising a cavity and a cover. A release film having good stretchability is placed between the intermediate and the cover, the release film having a higher resistance to dimensional change than the intermediate. Forming is effected indirect-y, that is, the compressed fluid acts on the release film, and the latter forms and presses the intermediate against the mold cavity. The release film has a higher resistance to dimensional change and resists such change to a greater degree than the intermediate. Under these conditions, a pressure of between 2 and 20 bars is preferably provided with the gaseous or liquid pressurized fluid. Application of pressure and temperature preferably lasts until the soft thermoplastic intermediate and the release film have molded themselves against the mold contours.